The advancement and development of plastic materials has led to the design and implementation of composite outer body panels for vehicles as well as composite cargo boxes for pickup trucks. Each of these composites requires different performance characteristics for their particular application. For an outer body panel of a vehicle, a reinforced reaction injection moldable (RRIM) material is injection molded to provide an outer body panel with certain desired properties, such as a class "A" surface with a medium flex modulus and good impact performance. For a pickup truck cargo box, a structural reaction injection moldable (SRIM) material is utilized to provide the rigidity and strength that is needed for a pickup truck cargo box as well as providing a material that has good flexibility and impact performance. To provide the required strength in the pickup truck cargo box, the SRIM material has a high glass strand content whereas the RRIM material has a low glass strand content. The high glass content of the SRIM material creates a low coefficient of linear thermal expansion while the lower glass content of the RRIM material creates a high coefficient of linear thermal expansion. The higher coefficient of linear thermal expansion of the RRIM material means that the outer body panel will thermally expand and contract more than the lower coefficient of linear thermal expansion of the pickup truck cargo box thereby causing relative movement between the aforementioned members when secured to each another. If the outer body panel is rigidly secured to the underbody and not allowed to expand and contract upon linear thermal expansion and contraction, internal stress will build in the weaker RRIM material of the outer body panel causing the outer body panel to be distorted and damaged. Linear thermal expansion is a function of the length of the material where over a six foot (1.82 meters) outer body panel, a size difference of 0.32 inches (8.0 millimeters) will occur at 150 degrees Fahrenheit (65.5 degrees Celsius) between the RRIM outer body panel and the SRIM underbody. Therefore, thermal expansion and contraction will be much greater over the longer length of the outer body panel than the shorter height of the outer body panel.